1. Field of the Invention
The present invention relates to an oscillator circuit and integrated circuit for oscillation using a piezoelectric resonator such as a crystal resonator, particularly relates to the oscillator circuit and integrated circuit for oscillation to which a circuit for neutralizing an equivalent parallel capacitor of the piezoelectric resonator is added.
2. Description of the Prior Art
A typical Colpitts-type crystal oscillator circuit employs a constitution in which, for example as shown in FIG. 3, the crystal resonator 31 is connected between an input and output terminals of an inverting amplifier 32 for amplifying the oscillation and a load capacitor 33 is connected to either end of the crystal resonator 31 for the oscillation.
In the Colpitts-type oscillator circuit shown in FIG. 3, the crystal resonator 31 is shown like FIG. 4A in an equivalent circuit, and the entire oscillation circuit is shown like FIG. 4B in the equivalent circuit. The crystal resonator 31 comprises the equivalent parallel capacitor C0, a resistance R1 (crystal impedance), an equivalent series capacitor C1, and an inductor L1. The oscillation circuit comprises a negative resistance xe2x88x92R, a resistance Re as an effective resistance of the crystal resonator, a capacitor CL comprising the load capacitor, and the inductor L.
When the equivalent parallel capacitor C0, resistance R1, capacitor CL, and resistance Re are expressed in respective symbols, the resistance Re is given as follows:
Re=R1xc2x7(1+C0/CL)2 
The oscillation continues as long as the sum of the resistance Re and negative resistance xe2x88x92R in the equivalent circuit of FIG. 4B is a negative value. However, while the equivalent parallel capacitor C0 and resistance R1 tend to increase with increase of frequency of the oscillation in the oscillator circuit of FIG. 3, the negative resistance xe2x88x92R in the entire circuit decreases in inverse proportion to the square of the frequency, resulting in difficult activation of the oscillation.
Moreover, when a varicap diode is connected parallel to the load capacitor 33 to form a voltage controlled oscillator circuit, in case the equivalent parallel capacitor C0 is large, frequency variation xcex94f/f0 with the capacitor CL being varied becomes small. When the equivalent parallel capacitor C0, equivalent series capacitor C1, and capacitor CL are expressed in respective symbols, the variation xcex94f/f0 is given as follows:
xcex94f/f0=C1/(2xc2x7(C0+CL)) 
To solve these problems, there is an attempt to neutralize the equivalent parallel capacitor C0. As a method, it has been commonly done in the prior art that an inductive reactance (coil) is added parallel to the crystal resonator. However, the method has a drawback that volume of the component is too large. Moreover, it is difficult to integrate the coil on the integrated circuit except for a special case. As the circuit for neutralizing the equivalent parallel capacitor, there is a widely used circuit of adding a feedback capacitor to compensate an input capacitance when a differential amplification circuit is used. However, since the circuit is realized by using a differential amplifier, it involves a significant delay to be disadvantageous in speeding up for application to the oscillator circuit, and a complicated constitution.
Therefore, the invention intends to achieve the neutralization of the equivalent parallel capacitor C0 of the piezoelectric resonator in a simple constitution to avoid increase of the effective resistance Re of the piezoelectric resonator and keep the value of Re sufficiently smaller than that of the negative resistance xe2x88x92R to activate the oscillation stably, and secure a sufficiently large variation xcex94f/f0 by neutralizing the equivalent parallel capacitor C0 when the voltage controlled oscillator circuit is formed.
The oscillator circuit of the invention, which is the Colpitts-type oscillator circuit having the piezoelectric resonator such as the crystal resonator and a first inverting amplifier having the input and output terminals connecting the piezoelectric resonator between them, is characterized in having a Miller capacitor circuit, which comprises a second inverting amplifier having an input side connected to the output terminal and an output side connected to the input terminal through the capacitor, for electrically neutralizing the parallel capacitor existing equivalently between both sides of the piezoelectric resonator.
Preferably, the second inverting amplifier comprises a bipolar transistor and determines a gain using a resistive divider.
Also, the second inverting amplifier preferably comprises a CMOS inverter and determines the gain using the resistive divider.
Also, the oscillator circuit is preferably integrated in a single integrated circuit for oscillation except for the piezoelectric resonator.